The present invention relates to a method and a device for clearing yarn, in which properties of the yarn are acquired and yarn defects to be removed are defined by means of an adjustable clearing limit.
A method for clearing yarn defects with the use of adjustable limits is disclosed, for example, in CH 683 350. In this method, yarn defects are displayed and classified two dimensionally on the basis of a deviation from a setpoint value of the yarn thickness and the length of the yarn defect. The numbers of yarn defects, which have been identified and measured, are entered in a two-dimensional classification field and stored, for example, in cells. The clearing limit is adjusted in such a way that it is shifted outwards, i.e. made less restrictive, in the vicinity of cells having high numbers of yarn defects, and inwards in the vicinity of cells having low numbers of yarn defects. In this manner, the number of necessary knots or splices in the yarn is reduced.
Such a method allows the clearing limit to be positioned in any desired manner so that it may assume any desired shape. However, the sensitivity limit is set manually, rather than in an adaptive manner. Consequently, this method entails costly experiments on a yarn, which have to precede yarn production or rewinding of the yarn.
Another method for adjusting operating limits of electronic yarn clearers is disclosed in CH 681 462. In this case, during the clearing process the measured values of the count are continuously recorded and their distribution is determined. On the basis of this distribution and a preselected permissible alarm frequency, the operating limits are automatically fixed in accordance with statistical regularities.
This further method relates to the adjustment of operating limits in yarn monitoring installations where yarn count deviations, or deviations of the yarn fineness i.e. of the mean dimension of a yarn, trigger an alarm or stop production. It therefore does not relate to the response of the yarn clearers to measured and varying yarn properties. Thus, the operating limits have nothing to do with short but extreme deviations of the yarn diameter. These operating limits are independent of any lengths.
A method of achieving optimum management of a clearing limit without a high outlay therefore has not yet been provided.
The present invention achieves the object of providing a method and a device which enable the fixing and adjustment of the clearing limit for yarn clearers to be improved in such a way that an optimum adjustment may be achieved as frequently as possible, while simultaneously satisfying specific stipulations.
This object is achieved by automatically fixing the clearing limit on the basis of the acquired properties. The clearing limit, once fixed, is preferably also automatically adjusted at the yarn clearer so that it may adapt periodically or continuously to the nature and frequency of the yarn defects which arise. This may be effected on the basis of a standard or initial adjustment or on the basis of data acquired from previous production of the same article. Fixing of the clearing limit is, in this case, the result of a closed-loop control, which takes into account the measured values of properties of the yarn and various important criteria for the characteristic of the clearing limit. These criteria may be difficult to measure or may be impossible to bring into a clear mathematical relationship with the clearing limit. In a preferred embodiment, therefore, the afore-mentioned criteria are processed according to the rules of fuzzy logic. For fixing, the values of yarn defects are acquired by, for example, yarn clearers at the yarn and classified according to measured parameters in that they are filed in a classification field and modeled in accordance with preselected assumptions about yarn defects. The density of the yarn defects in the classification field is determined from the modeled yarn defects. Criteria regarding the position of the clearing limit are derived from this density.
A device according to the invention substantially comprises a control loop, having a fuzzy loop controller, an input for values of properties acquired from the yarn and units for the entry of criteria for determining or influencing the clearing limit. A control loop may alternatively comprise a plurality of inputs for values of a plurality of yarns and may be connected to a plurality of yarn clearers for outputting a common clearing limit.
Among the advantages achieved by the invention, a wide range of criteria for fashioning the clearing limit may be taken into account. These criteria may relate to the yarn, e.g. to the density of the yarn defects or to the form of the yarn package, or they may relate to the installation at which the yarn is produced or rewound, e.g. to the type of sensor (optical or capacitive). Further criteria may take into account general quality considerations such as, for example, the fact that large yarn defects are more serious than small ones or that specific defects in one region are extremely serious for the user and so on. It is equally possible for clearing limits to be adapted to the method used to measure the yarn defects. For instance, it is possible to take account of the fact that capacitive sampling of the yarn no longer fully detects very short yarn defects, whereas optical sampling detects even short yarn defects to their full extent. It is therefore possible to ensure that a yarn that is cleared with the use of optical sampling is not spliced or knotted more often than a yarn which has been capacitively sampled. The system may operate both autonomously, i.e. without any special input, on the basis of a standard initial input or it may, as a result of suitable inputs, operate in an optimized manner according to all possible desirable criteria. By virtue of the proposed modeling of the yarn defects on the basis of determined yarn defect values, it is possible to reduce the quantity of samples or yarn defect values which are necessary for producing a representative relief of the yarn defect density, and hence for fixing a clearing limit.